Not applicable.
Not applicable.
1. Field of the Invention
This invention relates to a process for converting methoxy compounds, like methanol or dimethyl ether, into olefins, preferably ethylene, by contacting such methoxy compounds over a series of fixed catalyst beds.
2. Description of the Related Art
A new family of molecular sieve catalysts has been developed by Union Carbide workers. See U.S. Pat. No. 4,499,327. These silico alumino phosphate molecular sieve catalysts have demonstrated a high activity for converting methoxy compounds like methanol and/or dimethyl ether into olefin mixtures. Unlike earlier ZSM-5 catalysts which convert methoxy compounds to liquid grade hydrocarbons of a gasoline range (methanol to gasoline; or MTG), these new molecular sieve catalysts for conversion of methanol to olefins are more selective to the conversion of methanol to C2-4 olefins (MTO) and produce a lower concentration of higher carbon number alkylate and aromatic by-products. Based upon the carbon content of the feedstock, these new methanol to olefin (MTO) catalysts produced high yields of C2-4 olefinic materials.
Even so, with the new (MTO) catalysts, the formation of aromatics (C6+) is not completely suppressed. Because of the molecular size of aromatic by-products, once formed, they can not readily pass through the pore structure of the Zeolite catalyst. Such aromatics as are formed, which are thus effectively trapped against further free passage through the catalyst pores, are believed to undergo further reactions to ultimately yield coke. Hence, even with the new (MTO) catalyst, if used in a fixed bed operation, such catalyst has a very short active life before regeneration of the catalyst becomes necessary. Time on stream (TOS) of a fixed bed of MTO catalyst was initially measured in hours only. Such a short operational time on stream, obviously, is not acceptable as a basis for a commercial operation; because of the need to regenerate the catalyst after its activity has deteriorated. For safety reasons, when a catalyst reaction vessel is taken out of service for regeneration of the catalyst therein, the process shutdown period must be much longer than the few hours needed to simply regenerate the catalyst.
Many factors have been considered in an attempt to prolong the useful catalyst life of the new MTO catalyst for use in a fixed bed operation. Lowering of the partial pressure of the methoxy compound in contact with the MTO catalyst was examined as one possible means of prolonging the useful active catalyst life. Although lowering the partial pressure of methanol over the MTO catalyst was found to prolong the period of time for which the MTO catalyst was active, it was also discovered that the rate at which methoxy compound converted over the MTO catalyst was also reduced as a function of reducing partial pressure of the methoxy compound in essentially an inverse relationship to the period of time by which the MTO catalyst activity was prolonged. The end result of this inverse relationship is that the total amount of olefin product make during the increased TOS (time on stream) of the catalyst remained essentially unchanged. That is, the total quantity of olefin product made between catalyst regenerations (the TOS period) remains essentially the same whether the time on stream (TOS) was short because of a high initial partial pressure of the methanol feed or long because of a low initial partial pressure of the methanol feed. In other words, the degree to which the methanol feed to a one pass single fixed bed of MTO catalyst was diluted by a non-reactive component, such as steam or a non-reactive hydrocarbon, was found to be an ineffective parameter by which to increase or control the final yield of olefin product during the on steam operational time of the MTO catalyst.
While market conditions may vary at any particular point in time, on average ethylene is a much more valuable product than other higher olefins like propylene or butylene. Hence, it is of a long run concern that the yield of ethylene as based upon carbon feed input is maximizable compared to that input carbon which is diverted into higher olefins like propylene or butylene. With the new MTO catalyst, it has been demonstrated that ethylene is the product olefin first made upon initial contact of methoxy compound with the catalyst, but that on further contact of the ethylene make with the catalyst, the ethylene initially made is converted into other olefins like propylene and/or butylene. In a one pass process, wherein methoxy compound is passed only one time over a fixed catalyst bed at a high conversion rate of the methanol, the by-reactions which deplete the initial ethylene make by conversion of same to other higher olefins become most pronounced at high methanol conversion levels; that is when methanol conversion exceeds about 90%.
Given the so demonstrated short TOS of the new MTO catalyst at high conversion rates that equate to acceptable olefin yield rates, when considered for fixed bed operations, an inclination would be to use a high MTO catalyst inventoryxe2x80x94much more than the minimum quantity of MTO catalyst required for an acceptable methanol conversionxe2x80x94so that upon aging of the MTO catalyst mass from the inlet to outlet side of the reaction vessel there will remain a MTO catalyst zone of sufficient activity to continue operation at acceptable conversion rates of the feed MeOH. That is, the mass-volume of the MTO catalyst inventory undergoes a progressive volume zone aging which proceeds from the feed input, wherein methoxy compound partial pressure is at its highest level, and thus coking is at its highest rate, to that point within the catalyst mass volume inventory at which there is essentially no methoxy compound partial pressure because of its essentially complete conversion at this point within the catalyst mass (about 90% of methanol conversion), after which no significant coking occurs in the catalyst mass after this point. This zone of aging catalyst moves progressive through the fixed catalyst bed and as the leading edge of this zone becomes catalytically inactive due to its high coking level, until such time as that zone of catalyst closest to the product outlet side of the reactor has, through the width of its mass of this zone, undergone such aging/coking as to reduce its activity below acceptable levels. At this point in time, a fixed bed reactor of the MTO catalyst would be taken out of operation and its entire inventory of MTO catalysts would have to be regenerated to restore its activity. This, today, is the only expedient for utilizing the new MTO catalyst in a fixed bed operation such that the reactor could possibly operate for an acceptable period of time at practical olefin production rates. However, as one will readily appreciate, there is a high capital cost involved in this expedient for extending TOS. Further, in this single bed-one pass-high conversion-mode of operation there is no capability for controlling the degree of methanol conversionxe2x80x94the degree of conversion will always by high, xe2x89xa795%, and typically about 99%. As previously noted, at such high degrees of methanol conversion significant amounts of the ethylene make is diverted by side reactions into production of propylene and/or butylene.
Further, with fresh, or freshly regenerated catalysts, as would be the case with the regeneration of that mass of MTO catalyst within a one pass single fixed bed, the initial ethylene production of the regenerated MTO catalyst is very low. It was found that as a reasonable amount of coke is deposited on the catalyst, the ethylene yield is actually increased substantially. A. N. Rene Bos et al. Conversion of Methanol to Lower Olefins, Ind. Engl. Chem. Res., 1995 Vol. 34, pp. 3808-3816. The coke formed at high temperature on such MTO catalysts is most likely due to the formation of small amounts of aromatics which, becoming entrapped in the small pores of the catalyst, become further highly methylated at high temperatures. A further coke forming process of consequence to the MTO catalyst is that of polymerization of olefins at lower temperatures.
Given all of the shortcomings of the new MTO catalyst it is not surprising that at an early point in time the Union Carbide researchers, who developed this particular MTO catalyst, discounted the possibility of proceeding practically with a fixed bed operation with this catalyst and, instead, switched their attention to a fluidized bed process with this MTO catalyst. A fluidized bed operation for utilization of this MTO catalyst has also been pursued by UOP and Norsk Hydro.
A fluidized bed operation overcomes a further drawback of this MTO catalyst, that being one of a high pressure drop occasioned by the fine power nature in which the new MTO catalyst is formed.
It is desirable to devise a process which would allow the new MTO catalyst to be utilized in a fixed catalyst bed procedure, especially if its utilization in a fixed bed mode could provide a higher ethylene yield than is presently possible with those fluidized bed operations now proposed.
This invention comprises a process for converting methoxy compounds in the presence of a diluent, preferably steam or steam and/or a non-reactive organic compound, by passing the methoxy feed compound into multiple contacts with fixed beds of MTO catalyst in a series of fixed bed reactors. The feed rates of the methoxy compound is such, that in each of the reactors in series only a partial reaction of the methoxy compound is taking place to at least 80% of the total methoxy amount fed to that reactor. The advantage of only feeding a fraction of the total methoxy compound to a single reactor stage is that in that reactor the potential adiabatic temperature rise is reduced considerably. In the following reactors the methoxy compounds can be fed at lower than reaction temperature, thus reducing the inlet temperature of the total gas mixture into that reactor stage. A further reduction of the adiabatic temperature rise is possible by recycle of the effluent of a reactor stage.
Therefore, in a preferred version of the invention, a quantity of methoxy compound is added as fresh feed to each of the series of back-mixed fixed bed MTO catalyst reactors, wherein a portion of the product gas composition of this reactor is recycled back to the feed inlet of this particular reactor as a portion of its total feed and a remainder portion, or a bleed portion of the product gas composition of this, or any one reactor is passed as feed to the next in series, thus building up ethylene partial pressure through the series of reactors gradually, from stage to stage, while maintaining a low partial pressure of methoxy compound in any one of the series reactor.
In each back-mixed reactor stage the partial pressure of the total methoxy compound, measured as an equivalent to methanol, is kept at between 0.02 and 0.2 atmospheres absolute (ata). For purposes of this partial pressure calculation of methoxy partial pressure, one unit of dimethyl ether is counted as if it were instead two units of methanol. The total pressure in any reactor of the series is also low, between 0.5 and 3 ata. The employment of a low total reactor pressure makes it acceptable to utilize large surface areas for the MTO catalyst in each reactor, which eliminates the danger of too great a pressure drop across any reactor, especially with the large product gas recycle over each reactor stage. A large volume of reactor product gas recycled back through the feed inlet to that reactor stage is necessary in order to achieve a close approach to total back-mixing within that particular fixed bed reactor. In the back-mix with recycle product gas from a reactor to form the total feed to this particular fixed bed reactor, of the total methanol fed into contact with the MTO catalyst therein, only a small percentage of the total methanol undergoes conversion to olefin products, on the order of 6-10%. Yet, on the basis of fresh feed methanol added to such reactor, especially to those reactors after the first reactor, the conversion of methanol is high, on the order of at least 90%. By this sequence wherein, in each reactor of the series of the total methanol passed through such reactor the conversion is low but relative to the fresh methanol added to such reactor the conversion is high and of such the selectivity of converted methanol into ethylene is enhanced; while overall production of ethylene product by the sequence of two or preferably three or four back-mixed fixed bed MTO catalyst reactors is maintained to a comparable or greater level per unit time as that theoretically possible per unit time with a one pass-single fixed bed MTO catalyst bed. This while with the two, three and/or four fixed bed back-mixed sequence the TOS of the MTO catalyst inventory as a sum of this series of fixed bed reactors, is greatly enhanced such that the quantity of ethylene produced as a function of the MTO catalyst inventory TOS is thus greatly enhanced.